The peripheral sensory nervous system in the vertebrate head has a dull origin from cranial ectodermal placodes and neural crest cells. Placodes are discrete regions of thickened columnar epithelium on the head that share many properties with neural crest cells including the ability to undergo an epithelial to mesenchymal transition and migrate. Although the neural crest is being studied in depth, far less in known about placodes. We will explore the tissue and molecular interations that lead to placode formation and will use molecular markers for specific placodes to assess their origin and differentiation. We will primarily concentrate on the trigeminal placode, which develops adjacent to the neural tube, and the nodose placode, which develops at some distance from the neural tube. In addition, molecular markers for other placodes will be explored. An important component of this study will be to explore intrinsic similarities and differences between placodes and neural crest, in order to shed light on their embryonic relationships. Specifically, we will examine when placode and neural crest cell lineages segregate during development by determining how late in development a single cell can contribute to both placode and neural crest cell linages. We will study whether and to what extent placodes can form neural crest cell derivatives by examining their ability to interchange for one another. Finally, we will explore why the rostral neural folds from placodes but not neural crest cells. Specific experiments will: 1) examine the induction and development of the trigeminal placode. 2) examine the induction and development of the nodose placode. 3) identify molecular markers for other epibranchial placodes. 4) examine the interrelationships of the neural crest and placodes.